Seoul says it fired warning shots and broadcast warnings to expel a North Korean vessel that crossed a disputed maritime border.

The South Korean Joint Chiefs of Staff (JCS) said on Sunday that it fired warning shots and broadcast warnings to turn back at a North Korean patrol boat that breached the Northern Limit Line (NLL) at around 11 am Saturday (2:00 GMT).

“Our military maintains decisive battle posture while monitoring the enemy’s movements in preparation for potential provocations regarding NLL violations by North Korean patrol boats,” the JCS said in a statement.

During the operations, a South Korean patrol ship came into “minor contact” with a nearby Chinese fishing vessel due to bad visibility, resulting in no safety issues but slight injuries among the South Korean crew, the JCS added.

The reported North Korean incursion came as tension flared over Pyongyang’s stepped-up military activities in recent weeks, including Friday’s test of a new solid-fuel intercontinental ballistic missile, which experts say would facilitate missile launches with little warning.

Since the 1990s, Pyongyang has disputed the NLL – drawn up at the end of the 1950-53 Korean War – arguing it should lie far to the south.

Last October, the two Koreas traded warning shots in the western waters, accusing each other of breaching the sea border in an area where confrontations have often occurred.

Pyongyang has threatened military action as South Korea and the United States have carried out their annual springtime exercises since March, calling them a rehearsal for nuclear war.

North Korean leader Kim Jong Un this week ordered strengthening war deterrence in a “more practical and offensive” manner to counter what the isolated country called moves of aggression by South Korea and the US.

Seoul and Washington say their drills are defensive and aimed at deterring Pyongyang.

South Korean officials also say North Korea has not been responding to South Korean calls on a set of cross-border inter-Korean hotlines for more than a week, which raises concerns about potential kinetic provocations as communications on those channels are meant to prevent accidental clashes along the rivals’ sea borders.